1. Field of the Invention
The present invention relates generally to laboratory testing of reverse osmosis apparatus, and particularly to a device and method for testing reverse osmosis membranes that provides a static diffusion cell for measurement of the efficiency of flat membranes used for reverse osmosis.
2. Description of the Related Art
Reverse osmosis (RO) is a method that removes many types of large molecules and ions from solutions by applying necessary pressure to the solution when it is on one side of a selectively permeable membrane to reverse the normal osmotic process. The result is that the solute is retained on the pressurized side of the membrane and the pure solvent is allowed to pass to the other side. In order to be “selective”, the membrane should not allow large molecules or ions through its pores, but should allow smaller components of the solution, such as the solvent, to pass freely.
In the normal osmosis process, the solvent naturally moves from an area of low solute concentration through a membrane to an area of high solute concentration. At equilibrium, the concentration of the solute is equal on both sides of the membrane. The movement of a pure solvent to equalize solute concentrations on each side of a membrane generates a pressure, and this is the “osmotic pressure”. Applying an external pressure to reverse the natural flow of pure solvent, thus, is “reverse osmosis”. The process is similar to membrane filtration. However, it should be noted that there are key differences between reverse osmosis and filtration. The predominant removal mechanism in membrane filtration is straining, or size exclusion, so the process can theoretically achieve perfect exclusion of particles regardless of operational parameters such as influent pressure and concentration. Reverse osmosis, however, involves a diffusive mechanism so that separation efficiency depends upon solute concentration, pressure, and water flux rate. Reverse osmosis is most commonly known for its use in desalination processes to provide drinking water purification from seawater, removing the salt and other substances from the water molecules.
In reverse osmosis, pressure is applied to a compartment with high concentration. In this case, there are two forces influencing the movement of water: the pressure caused by the difference in solute concentration between the two compartments (i.e., the osmotic pressure) and the externally applied pressure. FIG. 4 diagrammatically illustrates a conventional reverse osmosis process of the type typically used in desalination. In system 100, seawater SW1 flows under pressure (generated by a high pressure pump 102) into a reverse osmosis unit 106 (with a reverse osmosis membrane mounted therein). Seawater SW1 is also drawn into a pressure exchanger 108.
Filtration within the reverse osmosis unit 106 generates both fresh water FW to be extracted, and also the waste concentrate flow CF, which is also pushed, under pressure, through the pressure exchanger 108. Waste concentrate is extracted and removed through concentrate drain CD, and now pressurized, mixed seawater SW2 is drawn by circulation pump 104 and injected back through the reverse osmosis unit 106.
Because reverse osmosis filtration is used in the production of drinking water and for other purposes in which purity and efficiency is critical, constant testing of the reverse osmosis membranes themselves is necessary. Particularly, in the field of desalination, salt rejection, scaling and fouling are all critical factors to be tested. Salt rejection is typically presented as a percentage, calculated as 100×[1−(product concentration/feed concentration)]. Scaling is the precipitation and deposition of scale forming compounds on the membrane surface when the brine is concentrated. Fouling is the deposition or accumulation of contaminants on the membrane surface.
Typical testing occurs dynamically in actual reverse osmosis processes, which are often performed in large-scale plants (particularly in desalination processes), thus requiring a great deal of effort. It would be desirable to provide small scale testing which is relatively quick and easy and provides highly accurate results.
Thus, a device and method for testing reverse osmosis membranes solving the aforementioned problems is desired.